Seamless handoff system and method

ABSTRACT

A multicellular communications system where transmission between a roving subscriber and multiple base station transceivers is maintained throughout the territory. Each mobile subscriber unit has global code seeds for the entire communications system or an updated neighbor list and continuously searches to access available cell base stations while maintaining a communication link with one base station. Candidate cell base stations are interrogated and communicated with. The candidate base station that requires less transmitting energy from the mobile subscriber unit is closely monitored. When the mobile subscriber unit can successfully communicate with less transmit power than currently required by the present base station, the mobile subscriber unit is handed off to the candidate base station transceiver.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to communication systems. Morespecifically, the invention relates to a system and method forcontrolling handoff between individual cells for a mobile subscriber ina multicellular communication environment.

2. Description of the Prior Art

Handoff is a general term describing the function of transferring acommunication signal for a mobile subscriber from one base stationtransceiver to another base station transceiver as the subscribernegotiates the cellular territory. There are two general methods ofconducting handoff in a cellular communication system, soft and hardhandoff. Quality may suffer using either handoff method as the mobilesubscriber switches base stations between individual cells.

Hard handoff typically occurs near the boundaries between cells. Throughcontinuous measurements of received signal power from a mobilesubscriber, the base station where the subscriber has establishedcommunication determines if the power is reduced below a nominal valuenear the cell boundary. The hard handoff to a candidate cell's basestation occurs instantaneously without disrupting the call in progress.The received power at the candidate cell's base station is much greaterthan required to ensure an ideal handoff. The difference between thecurrent cell's power and the candidate cell's power significantlyreduces capacity by interfering with other users.

A soft handoff occurs throughout a given range of distances from thecurrent and candidate cells' base stations. In the soft handoff method,the user is connected to both base stations as he travels near thecommon cell boundary. The decision to switch is made depending upon thereception of the mobile subscriber's pilot signal. A central switchingcenter decides at what point one of the base stations should be dropped.Shared communication is performed for a finite period of time, duringwhich transmission from the current and candidate cells is required.

In a typical CDMA communication system, the plurality of signals aretransmitted within the same frequency band. Frequency reuse not onlyapplies to users in the same cell, but also to those in all other cells.Since the same frequencies are used, the transmitted power levels fromthe mobile subscriber and the base station must be monitored closely. Ifpower control is not strictly adhered to, the overall transmissioninterference and the total number of usable channels is adverselyaffected. Therefore, the number of signals which can be successfullytransmitted and received is associated with the total power of allusers.

Both soft and hard handoff methods have shortcomings. Hard handoff canpotentially suffer from high drop-out rates. The soft handoff methodrequires a duplication of transmission resources from the current basestation and a base station from at least one candidate cell. The mobilesubscriber must establish two concurrent communication links, therebyrequiring twice the transmission power which would otherwise berequired. The increase in transmission power wastes power and aircapacity and contributes to total system interference while the handoffis taking place. The power or energy transmitted in a CDMA system byeach user must be kept at the minimum necessary to convey informationand to minimize interference with the other users. Careful control oftransmission power also contributes to extended use of portable devicesrelying on battery power. Furthermore, diversity combining of theduplicate signals at the central switching center is cumbersome unlessthe delay from both cell base stations is nearly identical.

Accordingly, there exists a need for an efficient, fast and reliablehandoff method.

SUMMARY OF THE INVENTION

The present invention relates to a multicellular communication systemwhere transmission between a roving subscriber and multiple base stationtransceivers is maintained. Each mobile subscriber unit has global codeseeds for the entire communications system and continuously searches toaccess available cell base stations while maintaining a communicationlink with one base station. Candidate cell base stations interrogate andcommunicate with the mobile subscriber unit. The candidate base stationthat requires the least transmit power from the mobile subscriber unitis closely monitored. When the mobile subscriber unit can successfullycommunicate with less transmit power than currently required by thecurrent base station, the mobile subscriber unit renders the decision tobe handed off to the candidate base station transceiver.

Accordingly, it is an object of the present invention to provide anefficient system and method for handoff between individual cells in amulticellular communications environment.

It is a further object of the invention to provide a system and methodpermitting the mobile subscriber unit to handoff between cells in amulticellular transmission system with reduced complexity andinterference.

Other objects and advantages of the invention will become apparent tothose skilled in the art after reading the detailed description of thepreferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a handoff system incorporating the present invention.

FIG. 2 is a handoff system incorporating the present invention.

FIG. 3 is a flowchart of the handoff method embodying the presentinvention when synchronized.

FIG. 4 is a flow chart of the handoff method embodying the presentinvention when not synchronized.

FIGS. 5A and SB are flow charts of the handoff method embodying thepresent invention.

FIG. 6 is the mobile subscriber unit employing the system and method ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is described with reference to the drawing figureswhere like numerals represent like elements throughout.

A system diagram illustrating the handoff system and method 15 of thepresent invention is shown in FIGS. 1 and 2. Four cells 17, 19, 21, 23of a multicellular telecommunications system 15 are shown with theirrespective base station transceivers 17', 19', 21', 23'. For clarity,none of the individual cells have been sectorized. A mobile subscriberunit 25 is shown near a common boundary between two of the individualcells 17, 21.

In operation, assuming that the mobile subscriber unit 25 iscommunicating with the first base station 17', as the mobile subscriberunit 25 moves from one cell location to another, the mobile subscriberunit 25 continues to communicate with the first base station 17' whilethe mobile subscriber unit 25 performs an analysis of the transmissionsof other base stations 19', 21', 23' within the system 15. At thispoint, only one traffic channel has been established between the mobilesubscriber unit 25 and the base station 17'.

In operation, the mobile subscriber unit 25 of the present inventionconstantly receives transmissions from base stations 17', 19', 21', 23'and others within communicating range of the mobile subscriber unit 25.The mobile subscriber unit 25 may continuously search to find a newcommunication link which requires less transmitting power, or searchonly when the transmitting power to the existing link approaches athreshold near maximum. In order to establish a new communication link,the mobile subscriber unit 25 must first synchronize with a candidatebase station transceiver. Synchronization is the process whereby themobile subscriber unit 25 must align its locally generated pseudorandomcode with the phase shifted code received from the candidate basestation. The transmission from the candidate base station includes ashort code, which is transmitted by the candidate base stationtransceiver and is used by the mobile subscriber unit 25 to determinephase ambiguity for synchronization. The mobile subscriber unit 25locally generates a replica of the global pilot that is transmitted fromthe base station. The mobile subscriber unit 25 then correlates thereceived signal with the locally generated replica of the global pilotcode. If the received and locally generated pilot codes are aligned, thecode phase has been found, otherwise the mobile subscriber unit 25changes the phase of the local code and continues the search until amatch is found. The mobile subscriber unit 25 performs the search from aset of global code seeds stored in memory to determine if the receivedpilot code matches one of the global code seeds. The memory of themobile subscriber unit 25 may contain a small "neighbor list" or it mayinclude all of the global codes used in the system 15. As shown in FIG.2, the neighbor list may include six cells 19, 21, 23, 27, 29, 31 in thefirst tier and twelve cells 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53,55, in the second tier. The neighbor list typically includes the nearest18 global codes to the existing link base station since the mobilesubscriber unit 25 will most likely transition to one of these cells.

Global seeds are assigned to base stations and are used in a basestation's service area to spread the global channels. Base stations arealso given primary seeds from which the base station generates assignedseeds. These assigned seeds are assigned to individual links between thebase station and the subscriber units and are used to spread assignedchannels. The global code seeds are preprogrammed within each mobilesubscriber unit 25. The mobile subscriber unit 25 can alternativelyacquire the global code seeds by periodically receiving an updated listbroadcast from each base station transceiver in the system 15.

The mobile subscriber unit 25 receives the plurality of transmittedshort codes from other base stations and uses them to accelerate theacquisition of the global pilots from the base stations. The mobilesubscriber unit 25 continuously searches all global code seeds in memoryuntil it finds a match by checking the short codes broadcast by eachbase station. The mobile subscriber unit 25 only has to examine theshort code to determine the global code for the cell. If the mobilesubscriber unit 25 does not recognize a short code, it aborts an attemptto synchronize to that base station and examines another received code.

When synchronization between the mobile subscriber unit 25 and thecandidate base station is complete, which will be described in moredetail later in the specification, the mobile subscriber unit 25transmits another symbol length short code while gradually increasingthe transmit power level. The mobile subscriber unit 25 monitors thecandidate base station for an acknowledgment signal, which acts as a"traffic light" to determine if the base station receives andacknowledges the short code. The transmit power ramp-up ceases upon twoconditions: 1) if the short code transmit power is within apredetermined switching differential for a finite period of time whencompared with the current power level of the pilot to the existing basestation; or, 2) the candidate base station acknowledges the detection ofthe short code and informs the mobile subscriber unit 25 via the trafficlight. The switching differential between the power levels of theexisting link and the candidate link is to prevent toggling back andforth between two base stations when both transmitting power levels areclose in magnitude. If the short code transmission requires a highertransmission power level than the current pilot power level, no handoffoccurs.

The handoff process does not require that the base stations besynchronized. However, if the base stations are synchronized, it iseasier for the mobile subscriber unit 25 to detect and synchronize to acandidate base station. If the base stations are not synchronized, thebase stations transmit a short sync code in addition to the globalpilot. The mobile subscriber unit 25 employs a multiple stageacquisition process where the mobile subscriber unit 25 first acquiresthe short sync code and then uses the phase information obtained toaccelerate the acquisition of the global pilot. In a system where thebase stations are synchronized, there is no need for the short synccode, the mobile subscriber unit 25 can acquire the global pilot quicklysince the relative phase difference between pilots from neighboring cellbase stations is small.

Referring now to FIGS. 3, 4, and 5A-5B, a flowchart illustrating thepreferred method with either synchronized or non-synchronized basestations is shown. When the individual cell base station transceiversare synchronized (as shown in FIGS. 3, 5A-5B), the process 15 begins(step 101) by receiving a sequence of short codes broadcast fromavailable cell base station transceivers within the communicating range.The set of stored global code seeds within each mobile subscriber unit25 (step 103) are used to generate the short codes and are comparedagainst the received short codes broadcast from a candidate cell basestation (step 105a). The mobile subscriber unit 25 can acquire theglobal pilot signal from a candidate base station transceiver in lessthan 10ms while maintaining a current communication link. To acquire apilot signal from a non-synchronized candidate cell base stationtransceiver could approach 3 seconds. Since the non-synchronous case isunacceptably long, a multilevel search is undertaken to synchronize inthe minimum amount of time.

If the cell base stations are not synchronized (as shown in FIGS. 4,5A-5B), the mobile subscriber unit 25 (step 101) compares the shortcodes generated from the stored global code seeds (step 103) against thereceived short code broadcasts from candidate cell base stations. Themobile subscriber unit 25 searches through the stored set of global codeseeds (step 105b) until a match is found with one that belongs to thebase station transceiver serving that particular cell. If a mobilesubscriber unit 25 cannot find a match to the broadcast short code (step107), the mobile subscriber unit 25 does not continue with the shortcode from that particular base station and tries another (step 109).

If a match is found (step 107), the mobile subscriber unit 25 performs asearch to acquire a fast broadcast channel (step 111). The mobilesubscriber unit 25 uses the phase information obtained from the shortsync code to resolve the phase ambiguity of the global pilot signal andto determine the fast broadcast channel. If the search is not successful(step 113), the mobile subscriber unit 25 tries another global code seed(step 109). The mobile subscriber unit 25 again searches to acquire theglobal pilot signal (step 115). If the global pilot signal cannot beacquired (step 117) from the short sync code, another global seed istried (step 109).

Once the mobile subscriber unit 25 acquires a global pilot signalcorresponding to a synchronized or non-synchronized candidate cell basestation transceiver (step 117), the mobile subscriber unit 25 transmitsa symbol length short code to the candidate base station whileramping-up the transmit power level (step 119). As the power levelslowly increases, the mobile subscriber unit 25 monitors thetransmitting power (step 121) and the reverse traffic light from thecandidate base station (step 123) to determine if the candidatetransceiver receives and acknowledges the transmitted symbol lengthshort code. The transmission of the short code from the mobilesubscriber unit 25 ceases when either the short code transmit power iswithin the predetermined switching differential comparing the powerlevels of both communication links, or when the candidate cell basestation transceiver acknowledges the detection of the transmitted shortcode by observing the traffic light (step 123).

If the mobile unit maintains and searches through the whole list ofglobal codes rather than the neighbor list, very often the tested codewill not belong to a base station in close proximity. For example, therecan be a total of 64 global codes, but a mobile may receive only a fewof them from the neighbor base stations.

The decision to handoff is controlled by the mobile subscriber unit 25which requires less air capacity during handoff than the methodsdisclosed in the prior art. Since the decision to handoff is controlledby the mobile subscriber unit 25, the system 15 is not fixed as in theprior art. The system 15 is flexible with regard to cell boundarieschanging in reaction to air capacity within the individual cells.

The mobile subscriber unit 25 establishes a link to the cell basestation that requires the minimum transmit power from the mobilesubscriber unit 25 to maintain a communication link. The mobilesubscriber unit 25 sends a long access pilot (step 125) to the chosencandidate cell base station including an instruction that is furtherconveyed to a radio distribution unit. The radio distribution unit keepsrecord of which base station each mobile subscriber unit is currentlyassociated with. The radio distribution unit routes the communicationlink from a land line to the appropriate cell base station transceiveras the mobile user negotiates through the various cells. The messagesent indicates a handoff.

For this brief moment, the mobile subscriber unit 25 is still linked tothe current cell base station transceiver (step 127). The communicationlink to the original cell base station transceiver controls the transmitpower of the mobile subscriber unit 25. However, the candidate cell basestation transceiver is also sending power control commands. The powercontrol commands from the candidate cell base station transceiver lowerthe transmitting power of the mobile subscriber unit 25 in dependenceupon the candidate communication link. The mobile subscriber unit 25abandons (step 129) the first communication link to the current cellbase station transceiver and resumes power control and communicationfrom formerly the candidate, now the sole link to a cell base stationtransceiver (step 131).

A system incorporating the method is shown in FIG. 6. As describedabove, the method requires two separate communication links: 1) analternative link to search for new candidate base stations 61a, 63a, and2) a preexisting communication link 61b, 63b. The mobile subscriber unit25 includes a receiver 65 that can despread 67a, 67b at least twodifferent channels. Maintaining symmetry is a transmitter 69 which canspread 71a, 71b at least two channels associated with the despreaders67a, 67b. While a preexisting communication link is maintained, themobile subscriber unit 25 receives 73 available broadcasts fromcandidate base stations.

The present invention stores the seamless handoff process code onboardin ROM 75 and executes the code in a highspeed microprocessor 77 thatcontrols the despreaders 67a, 67b and spreaders 71a, 71b such that theglobal code seeds can be stored and recalled for the immediateneighboring cell sites from RAM 79 to constantly acquire an alternativecommunication link.

The preexisting communication link 61b, 63b includes voice processing81, the discussion of which is beyond the scope of this disclosure. Themicroprocessor 77 interrogates 83, 85 the receiver 65 and transmitter 69for monitoring the reception of reply codes from the candidate basestations and also to compare transmission power during the ramping-upstep. The process is preprogrammed in ROM 75 and loaded and executed inthe microprocessor 77 continuously when the mobile subscriber unit 25 isin operation. When a communication link is found that requires lesstransmission power, the microprocessor 77 switches 91 to the alternativelink 61a, 61b maintaining constant voice and data transmission whilepreventing two simultaneous data communications.

The mobile subscriber unit 25 seeks a communication link requiring lesstransmitting power and is always comparing a prospective communicationlink with the preexisting communication link. At no point during theprocess are two communication links established transmitting data. Theswitching is instantaneous between the current cell base stationtransceiver and the candidate cell base station transceiver.

As described, the above pertains to a cellular network which has notbeen sectorized. If sectorization is employed, which effectivelymultiplies the number of cell base stations, the handoff method of thepresent invention must be accomplished in much less time. When handingoff within sectors of the same cell, the base stations serving thedifferent sectors within the cells are synchronized and a limited searchprocess is performed. A small portion of each global code is searchedbefore doing a comprehensive search encompassing the entire global code.This allows the mobile subscriber unit 25 to acquire a candidate basestation in less time.

While the present invention has been described in terms of the preferredembodiment, other variations which are within the scope of the inventionas outlined in the claims below will be apparent to those skilled in theart.

What is claimed is:
 1. A method for a mobile subscriber to control cellhandoff in a multicellular CDMA communication system while roving amongindividual cells, each cell associated with a base station whichtransmits a global pilot signal, where the subscriber has a preexistingcommunication link with one of the base stations, comprising the stepsof:a) receiving a plurality of transmissions from a plurality ofcandidate base stations with which the preexisting link is notestablished; b) selecting a received transmission associated with acandidate base station; c) attempting to acquire a global pilot signalfrom said selected transmission; d) if said global pilot cannot beacquired, selecting a received transmission associated with a differentcandidate base station; e) repeating steps b-d until a global pilotassociated with a selected candidate base station is received; f)ramping transmitter power of the mobile subscriber while transmitting ashort access code from the mobile subscriber to said selected candidatebase station; g) monitoring transmit power to said selected candidatebase station; h) detecting a return transmitted signal from saidcandidate base station; i) comparing transmit power to said selectedcandidate base station with transmit power to the preexistingcommunication link; j) if said transmit power to said selected candidatebase station is equal to or greater than the transmit power of thepreexisting communication link, repeat steps b-i, otherwise transmittinga long access pilot to said selected candidate base station andestablishing a new communication link; and k) abandoning the preexistingcommunication link.
 2. The method of claim 1 further comprising the stepof said new communication link becoming the preexisting communicationlink.
 3. The method of claim 2 wherein the step of selecting a receivedtransmission further comprises the steps of:a) selecting a global codeseed associated with one of said plurality of candidate base stationsfrom a plurality of global code seeds stored in a memory; b) searchingfor a global pilot using said selected global code seed; and c)performing said search to a maximum expected phase shift between thepreexisting communication link and said selected candidate base station.4. The method of claim 3 further comprising the step of synchronizingsaid transmission between the mobile subscriber and said selectedcandidate base station.
 5. The method of claim 4 wherein the step ofsynchronizing further comprises the steps of:a) selecting a global codeseed associated with one of said plurality of candidate base stationsfrom a plurality of global code seeds stored in a memory; b) acquiring ashort sync code from said candidate base station using said selectedglobal seed; c) if said short sync code cannot be acquired, selecting areceived transmission associated with a different candidate basestation; d) repeating steps b-c until a global pilot associated with aselected candidate base station is received; e) acquiring a fastbroadcast channel based upon phase information obtained from saidreceived short sync code; f) if said fast broadcast channel cannot beacquired, selecting a received transmission associated with a differentcandidate base station; g) repeating steps b-f until a global pilotassociated with a selected candidate base station is received; and h)acquiring a global pilot using phase information obtained from said fastbroadcast channel.
 6. A mobile subscriber system that controls cellhand-off in a multicellular CDMA communication system while roving amongindividual cells, each cell associated with a base station whichtransmits a global pilot signal, where the subscriber has a preexistingcommunication link with one of the base stations, comprising:means forreceiving a plurality of transmissions from a plurality of candidatebase stations with which the preexisting link is not established; meansfor selecting a received transmission associated with a candidate basestation; means for attempting to acquire a global pilot signal from saidselected transmission; means for selecting a received transmissionassociated with a different candidate base station if said global pilotcannot be acquired; means for ramping transmitter power of the mobilesubscriber while transmitting a short access code from the mobilesubscriber to said selected candidate base station; means for monitoringtransmit power to said selected candidate base station; means fordetecting a return transmitted signal from said candidate base station;means for comparing transmit power to said selected candidate basestation with transmit power to the preexisting communication link; meansfor transmitting a long access pilot to said selected candidate basestation and establishing a new communication link if said transmit powerto said selected candidate base station is less than the transmit powerof the preexisting communication link; and means for abandoning thepreexisting communication link.
 7. The system of claim 6 wherein saidnew communication link becomes the preexisting communication link. 8.The system of claim 7 wherein the means for selecting a receivedtransmission further comprises:means for selecting a global code seedassociated with one of said plurality of candidate base stations from aplurality of global code seeds stored in a memory; means for searchingfor a global pilot using said selected global code seed; and means forperforming said search to a maximum expected phase shift between thepreexisting communication link and said selected candidate base station.9. The system of claim 8 further comprising means for synchronizing saidtransmission between the mobile subscriber and said selected candidatebase station.
 10. The system of claim 9 wherein the means forsynchronizing further comprises:means for selecting a global code seedassociated with one of said plurality of candidate base stations from aplurality of global code seeds stored in a memory; means for acquiring ashort sync code from said candidate base station using said selectedglobal seed; means for selecting a received transmission associated witha different candidate base station if said short sync code cannot beacquired; means for acquiring a fast broadcast channel based upon phaseinformation obtained from said received short sync code; means forselecting a received transmission associated with a different candidatebase station if said fast broadcast channel cannot be acquired; andmeans for acquiring a global pilot using phase information obtained fromsaid fast broadcast channel.